arm: gic-v3-its: Implement of_xlate

arm: gic-v3-its: Implement of_xlate

Translate IRQs by implementing of_xlate() as required by
irq_get_by_index() to parse interrupt properties.

Map DT interrupts to ARM GIC interrupts as follows:

- Interrupt numbers ID32-ID1019 are used for SPIs
- ID0-ID15 are used for SGIs
- ID16-ID31 are used for PPIs

TEST: Booted on qemu sbsa-ref that has a GICV3.

Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-by: Moritz Fischer <moritzf@google.com>